Recently, in an apparatus powered by a battery, primary cells that cannot be recharged, such as manganese primary cells, are being gradually replaced by secondary cells that may be recharged. These cells, such as nickel cadmium batteries and small sealed lead batteries may be used repeatedly. However, the secondary cell may have lower energy density than the primary cell, and hence to obtain the same cell capacity as the primary cell, a larger secondary cell may be required Accordingly, the secondary cell has been developed with increased capacity. Lately, the lithium ion battery capable of a much higher capacity than that of the nickel cadmium battery or small sealed lead battery has been introduced. This cell uses cobaltin lithium in the positive electrode and various compositions of carbon in the negative electrode. The resulting cell has a capacity 250 to 300% greater than the conventional nickel cadmium battery.
The lithium ion battery is generally charged by the constant current and constant voltage method similar to that of the lead battery. That is, it is charged at a constant current rate until the cell voltage reaches a set voltage value, and then charged at constant voltage thereafter. However, if the voltage charged is a little lower than an optimum value, then the lithium ion battery is under charged, and if the voltage charged is a little higher than an optimum value then the battery is overcharged. More specifically, when charged at a voltage exceeding 4.1 V in the lithium ion battery of which negative electrode is natural graphite material, or when charged at a voltage exceeding 4.2 V in the lithium ion battery of which negative electrode is coke material, these cells may be overcharged. The performance of an overcharged lithium ion battery deteriorates more quickly than a nickel cadmium battery. In a worst case, the lithium ions in the battery are formed in an acicular crystal structure called dendrite, and precipitate as metal lithium. This may penetrate through the separator, used as a partition between the positive electrode and negative electrode of the battery, possibly leading to a short-circuit in the battery, smoke or fire.
On the other hand, the constant-current/constant-voltage battery charger may be realized by using a resistance or series regulator, but it is often realized using a chopper circuit to satisfy efficiency and heat generation concerns.
A conventional constant-current/constant-voltage battery charger is described below by referring to the drawings in FIG. 9, FIG. 10A, and FIG. 10B.
FIG. 9 is a circuit diagram showing a constant-current/constant-voltage battery charger of the prior art. In FIG. 9, power source 1 supplies electric power for a chopper circuit 2. Chopper circuit 2 controls the charging current and charging voltage. Cell voltage detection circuit 5 detects the voltage of battery 6, such as a lithium ion battery, to be charged. Voltage controlled oscillator (VCO) has a variable oscillation frequency and depends on the applied voltage. Amplifier 11 amplifies the detected cell voltage from the cell voltage detection circuit 5 which is delivered to the voltage controlled oscillator 9 as a control voltage. Current limiting resistor 12 limits the charging current during constant current charging.
FIG. 10A is a graph showing characteristics of cell voltage v and charging current i as a function of charging time t of a lithium ion battery using a constant-current/constant-voltage battery charger of the prior art. FIG. 10B is a magnified view of part A of FIG. 10A. In FIG. 10A and FIG. 10B, V1 is a set constant voltage charging value, i1 is a fixed constant current charging value, tc is a changeover point from constant current charging to constant voltage charging, and r is a ripple component of the cell voltage.
The operation of the constant-current/constant-voltage battery charger having such constitution as described in FIG. 9 and such characteristics as described in FIG. 10A and FIG. 10B, is described below. In the charger in FIG. 9, when the power source 1 is turned on it supplies electric power for a chopper circuit 2. The cell voltage detection circuit 5 detects the voltage of the lithium ion battery 6. This detected cell voltage is amplified by the DC amplifier 11 and provided to the voltage controlled oscillator 9. As a result, the voltage controlled oscillator 9 generates a signal having a certain frequency, and delivers it to the chopper circuit 2 as an oscillation signal. Chopper circuit 2 then starts to supply a charging voltage and a charging current. When the cell voltage of the lithium ion battery 6 is lower than the output voltage of the chopper circuit 2, the charging current is determined by the difference of the two voltages and the resistance value of the current limiting resistor 12, and charging is done at a nearly constant current i1. That is, the output voltage (charging voltage) of the chopper circuit 2 is controlled so that the charging current value may be a nearly constant current value i1 and the cell voltage value may become closer to the voltage value V1. Therefore, charging in this stage is constant current charging. As shown in FIG. 10A, at time tc, when the cell voltage value reaches a voltage value V1, namely when the cell voltage detection circuit 5 detects fixed voltage value V1, chopper circuit 2 is controlled to stop increasing the battery voltage, and charging is changed over to constant voltage charging.
In the constant-current/constant-voltage battery charger of the prior art, however, overcharging is likely to occur in the lithium ion battery due to the excessive movement of lithium ions due to ripple component r (see FIG. 10B) of the chopper circuit 2.
That is, in the constant-current/constant-voltage battery charger of the prior art, the cell voltage of the lithium ion battery 6 is detected by the cell voltage detection circuit 5, but the cell voltage includes the ripple voltage component in the voltage supplied from the chopper circuit 2. This is shown in the magnified view shown in FIG. 10B. Chopper circuit 2 supplies current at a specific phase angle in a cyclic period to a capacitor in the chopper circuit 2. The current charges the capacitor, and in turn the chopper circuit 2 can then supply output voltage. Generally, the operating frequency of the chopper circuit 2 is selected from scores of kHz to hundreds of kHz in order to enhance the efficiency of the switching element and reduce the size of the inductor. Therefore, the ripple component of the output voltage is filtered by the capacitor, but any ripple component residue remaining in the charging voltage is supplied to the lithium ion battery 6.
When the lithium ion battery 6 is overcharged by the ripple component, as mentioned above, metal lithium may appear in the negative electrode or dendrite may be generated, which may possibly lead to an unsafe condition. On the market, meanwhile, there is a strong demand for increased capacity of secondary cells, and a charger capable of charging the large capacity lithium ion battery 6 safely and securely is needed.